Seminar – Thomas Rossby – MISU/University of Rhode Island, USA

EVENT

An observer’s view of the meridional overturning circulation in the northeast Atlantic: Fluxes, flux divergences and conversions

Thomas Rossby, MISU/University of Rhode Island.

Name

Thomas Rossby
MISU/Graduate School of Oceanography, University of Rhode Island, USA

Title

An observer’s view of the meridional overturning circulation in the northeast Atlantic: Fluxes, flux divergences and conversions

Abstract

The meridional overturning circulation (MOC) is viewed as a key component in the climate of the North Atlantic and regions surrounding it. In this talk I’ll first review what is meant by the MOC (or AMOC for Atlantic-MOC), namely a two-dimensional collapse of the North Atlantic circulation onto a meridional-vertical plane. This has been a convenient way for modelers to summarize their results, but has had the unfortunate effect of forcing an oversimplified view of the ocean wherein it is presumed that variations in estimated MOC strength at one latitude is thought to be representative of the MOC as a whole. In models the principal overturning (densification) takes place in the 50-60° latitude range corresponding to the Labrador Sea. This ignores the far greater role the Nordic Seas play in the overturning process.

Two recently completed studies of velocity, temperature and salinity in the northeast Atlantic, one along a line between between Scotland and Cape Farewell (line 1) and the other along a composite line between Shetland, the Faroes, Iceland and the Greenland shelf 1200 km northeast of Cape Farewell (line 2) have given us accurate estimates of volume, heat and fresh water fluxes between Europe and Greenland. The heat fluxes across these two lines indicate that the heat flux towards the Nordic Seas is a factor 4 greater than towards the Labrador Sea. It is also known that the principal source of North Atlantic Deep Water is the overflow from the Nordic Seas with the Labrador Sea contributing to the lower branch of the MOC through entrainment. This suggests that the stability of the MOC is primarily governed by conditions in the Nordic Seas (including the Arctic), not the Labrador Sea. During the last glacial maximum the Nordic Seas branch of the MOC was shut down, but not that of the Labrador Sea!

We conclude with a discussion of our ongoing research, which focuses on using the magnitude and structure of these estimated volume, heat and fresh water fluxes between the two lines to constrain how much of this heat divergence is lost to the atmosphere vs. ice melt. Similarly, an accumulation of about 0.1 Sv of fresh water in this region is more than twice expected from net precipitation – minus evaporation. The rest must come from ice melt.